1 Field of the Invention
The present invention relates in general to a master medium cleaning method, and in particular to a method of cleaning magnetic transfer master mediums, which are used to perform magnetic transfers, or optical disk master mediums, which are used in manufacturing optical disks.
2 Description of the Related Art
A magnetic transfer method is a method comprising the steps of: bringing a master medium, on which a transfer pattern has been formed, into close contact with a slave medium, which has been provided with a magnetic recording portion for receiving the transfer, to form a conjoined body; and applying a transfer magnetic field to the conjoined body so as to transfer and record the magnetic pattern corresponding to the transfer data (such as servo signals) onto the slave medium. This magnetic transfer method has been described in Japanese Unexamined Patent Publication Nos. 63(1988)-183623, 10(1998)-40544, and 10(1998)-269566, for example.
The master medium employed in the magnetic transferred is formed of a silicon substrate or a glass substrate, for example, on the surface of which an uneven pattern composed of a magnetic body is formed by use of a photo lithography process, a sputtering process, an etching process, or the like. Further, magnetic transfer master mediums can also be manufactured by use of a lithography technology such as that used in the manufacture of semiconductors, or a stamping technology such as that used in the manufacture of optical disks.
In order to improve the quality of the magnetic transfer described above, it is necessary that the slave medium and the master medium be conjoin so that no gap whatsoever exists therebetween. That is to say, if there are contact deficiencies therebetween, regions on which a magnetic transfer can not be performed occur; if a magnetic transfer can not be performed, signal omissions occur in the magnetic data transferred to the slave medium and the signal quality thereof is reduced, and for cases in which the transferred data are servo signals, an adequate tracking function can not be obtained, whereby a problem arises in that the reliability is reduced.
However, when performing magnetic transfers according to the magnetic transfer method described above, the surface of the master medium becomes littered with foreign matter that becomes attached thereto through the repeated usage thereof. This foreign matter can consist of dust, fiber strands and other debris originating in the surrounding environment.
If a magnetic transfer is performed when there is dust or other debris attached to the surface of the master medium, adequate contact between the area of the surface of the master medium centering on the dust or debris attached thereto and the surrounding vicinity thereof and the corresponding area of the slave medium cannot be ensured; whereby a pattern of a predetermined signal level cannot be transferred, and the transfer quality becomes deteriorated thereby. By repeatedly bringing a master medium, onto the surface of which foreign matter has become attached, into close contact with slave mediums, the attachment strength of the foreign matter to the surface of the master medium is heightened; whereby deficiencies in pattern transfer of the same or greater magnitude are repeated during the performance of magnetic transferences to slave mediums subsequently brought into close contact with said master medium, and said foreign matter becomes the cause of multiple faulty products. Further, the surface of the master medium becomes deformed by this attached debris, and a problem arises in that the proper functionality thereof is lost.
In this regard, cleaning technology, wherein a cleaning disk to which a cleaning pad has been attached is rotated while in contact with the surface of the master medium to remove dust and debris that have become attached thereto has been proposed as described in Japanese Unexamined Patent Publication No. 2000-285637, for example.
However, in performing cleaning using cleaning technology such as that described above, for cases in which the transfer pattern formed on the data bearing surface of the master medium is provided as an uneven form, foreign matter becoming lodged in the depression portions thereof is difficult to remove. Further, there are cases such as those in which foreign matter originating in the surrounding environment becomes embedded in the depression portions, cases in which the surface of the master medium becomes marred or scratched due to foreign matter, and cases in which it is not possible to remove minute particulate foreign matter that has become adhered to the surface of the medium, giving rise to a fear that transfer deficiencies caused thereby will not be corrected due to the inadequacy of the cleaning process. In particular, because master mediums are expensive, from the standpoint of cost, it is advantageous that the master mediums be reusable and have a long life span.
Further, cleaning methods performed before the magnetic transfer process in order to remove foreign matter that has become adhered to the surface of the master medium, such as an ultrasound cleaning using a cleansing liquid, a wiping cleaning, or the like can also be considered. However, it is possible that the foreign matter removed thereby becomes re-adhered after performing a cleansing by one of these methods, that the removal of the foreign matter is insufficient, that a watermark occurs on the surface of the master medium when the cleansing liquid has dried, or that the surface of the master medium becomes marred or scratched due to abrasion by the wiping material, giving rise to a fear that these become the origin of damage to the master medium, and that transmission deficiencies occur in the transferred signal. That is to say, even if foreign matter adhered to the surface of the master medium is temporarily removed by use of a cleansing liquid, a wiping material or the like, there are cases in which foreign matter in the form of washing liquid remaining on the surface of the master medium, or foreign matter from the wiping material becomes attached to the surface of the master medium; in particular, it is not possible to remove some of the minute particulate foreign matter that has become adhered to the surface of the medium.
When performing a magnetic transfer, the master medium and the slave medium are put into close contact with each other and a magnetic field is applied to the mediums to magnetically transfer the shape of the pattern of the master medium onto the slave medium. If something remains between the master medium and the slave medium in this process, an air gap is formed between the mediums and transmission deficiencies occur. This was discovered by inspecting the master medium that was reused after washing by the above-mentioned cleaning method. That is, it was discovered that the number of surface defects was increased by the cleaning method. There are cases in which the signal omissions occurring due to foreign matter having become adhered to the surface of the master medium in the course of the above-described magnetic transfer are present from the initial stages thereof when slave mediums are exchanged one after the other and the magnetic transfer is performed a plurality of times in succession. It has been determined that this is due to foreign matter that has become adhered to the surface of the master medium during the manufacture thereof. Further, analysis of the attached foreign matter has shown that the foreign matter is composed of organic matter; the foreign matter has been determined to be formed of the same source material as the coating material used to form the protective layer provided over the pattern of the substrate of the master medium.
More specifically, as described above, a pattern corresponding to the transfer signal is formed on the surface of the master medium, and a magnetic layer is formed over said pattern; because the pattern forming process and the magnetic layer forming process are performed separately, in order to prevent the attachment of foreign matter or marring of the surface of the master medium in the interim between the performance of said processes, a protective coating formed of organic material has been applied over the pattern.
Then when the magnetic layer is to be formed, the protective layer is peeled away from the surface of the pattern formed on the substrate, and the magnetic layer is formed thereon by use of a sputtering method; however, even after the protective layer has been peeled off, a portion thereof remains on the surface of the pattern, and this remaining portion of the protective layer is aerially dispersed during the magnetic layer forming process and becomes attached to the surface thereof thereafter, whereby the occurrence of signal omissions due to contact deficiencies is brought about. Further, material of the protective layer remaining between the substrate and the magnetic layer causes a reduction of the contact property of both the substrate and the magnetic layer, which becomes a cause of the peeling away of the magnetic layer from the substrate, giving rise to a problem in the reduction in the durability of the master medium.
On the other hand, stamping methods employed in manufacturing optical disks are methods of forming a resin layer on the uneven pattern of the data bearing surface of a master medium (a so-called stamper) optical disk. Then, the surface of the optical disk, which comprises a resin layer having an uneven form corresponding to the transfer data, is coated with a thin film formed of a reflective material, and a protective layer is further provided.
Therefore, because the master medium optical disks are manufactured in a continuous manner wherein a cleaning process is not performed during the course of the manufacture thereof, after the number of disks manufactured exceeds one-thousand, foreign matter becomes adhered to the surface of the optical disks in the same manner as for the magnetic transfer master mediums; this leads to an increase in the error rate of the optical disks, whereby it becomes difficult to produce optical disks having a high degree of reliability.
In view of the forgoing points, cleaning processes such as an electrolytic degreasing cleansing or a ultrasound cleansing employing liquid detergent, or an ozone cleaning apparatus are employed to clean the master medium optical disk; however, there are cases in which it is not possible to remove particles which have become firmly affixed to the surface of the optical disk. It has been particularly difficult to completely remove foreign matter from the depression portions of the uneven pattern, which results in formational deficiencies.
The present invention has been developed in view of the forgoing problems, and it is an objective of the present invention to provide a magnetic transfer master medium cleaning method; wherein, the foreign matter that has adhered to the surface of the master medium and which has originated from the protective layer provided on the substrate of the master medium is removed, and a high quality magnetic transfer is capable of being performed to the master medium that has been cleaned thereby.
Further, it is a further objective of the present invention to provide a master medium cleaning method of cleaning a magnetic transfer master medium or a master medium optical disk; wherein, the foreign matter that has become embedded in the depression portions of the uneven pattern of the magnetic transfer master medium or the master medium optical disk, including the minute particulate foreign matter, is completely removed, and a high quality magnetic transfer is capable of being performed by the magnetic transfer master medium or production of optical disks is possible by use of the master medium optical disk, which have been cleaned thereby.
The master medium cleaning method according to the present invention is a method of cleaning a master medium comprising a substrate having a pattern formed thereon, and a magnetic layer formed over said pattern, implemented by cleaning the surface of the substrate on which the pattern has been formed, before the formation of the magnetic layer thereover, by use of a plasma discharge carried out in an evacuated reactive gas environment.
A protective layer formed of an organic material is formed over the surface of the substrate of the master medium whereon the pattern has been formed, and the material from said protective layer remaining thereon after said protective layer has been peeled away from said surface is removed by incineration.
The cleaning apparatus implementing the cleaning according to the present invention comprises: a chamber for housing the master medium, an evacuating means for reducing pressure in the chamber, a plasma discharging means for discharging plasma between the electrode of the chamber and the master medium, and a gas introducing means for introducing a reactive gas into the chamber; wherein, the plasma is discharged in the state wherein the chamber has been vacuumed and the reactive gas has been introduced thereto, whereby the foreign matter adhered to the surface of the master medium is burned off of the surface of the master medium by the plasma discharge.
Another master medium cleaning method according to the present invention is a method implemented during the manufacture of a master medium, wherein the surface of a substrate, on which a pattern has been formed, is cleaned by plasma discharge within an pressure reduced reactive gas environment. That is, the remnants of a protective film are burned off of the substrate by the plasma discharge within the pressure reduced reactive gas environment after the protective film is peeled off of the substrate, on which a pattern has been formed, and before the formation of a magnetic layer thereon.
Yet another master medium cleaning method according to the present invention is a method of removing the foreign matter adhered to the data bearing surface, on which an uneven pattern corresponding to data has been formed, of a magnetic transfer master medium or a master medium optical disk, comprising the steps of: providing a film over the data bearing surface of the master medium, and removing the foreign matter adhered to the data bearing surface of the master medium along with said film.
The film can be, for example, a carbon layer; wherein, when the cleaning process is to be performed, this carbon layer is removed by the use of plasma. Further, the film can be a polymer film; wherein, when the cleaning process is to be performed, this polymeric film is removed by the use of a solvent or plasma. The removal of the film can also be performed by the use of an ultrasound cleansing that causes the film to be peeled away from the surface of the master medium. The film can be formed on the surface of the master medium by use of any of a variety of film forming methods: a sputtering method of sputtering a material containing carbon onto the surface of the master medium; a film forming method employing a CVD; a vapor deposition method of depositing a polymeric material onto the surface of the master medium; etc. A favorable magnetic transfer property and film formation property can be ensured for at a film thickness of less than or equal to 40 nm. If the film is thicker than 40 nm, in the case that a magnetic transfer is to be performed, the distance between the surface of the pattern and the slave medium is increased, causing a deterioration of the transfer property and a reduction in resolution, and in the case that an optical disk is to be formed, the change in the disk formation dimensions becomes large.
For cases in which the magnetic transfer master medium or the optical disk master medium is to be reused after having been subjected to the cleaning process, the film is reformed on the respective data bearing surface thereof and the master medium is reused.
Further, in the case of the magnetic transfer master medium, the film can be the magnetic layer formed on the pattern surface of the substrate. When the cleaning process is to be performed, the magnetic layer is removed, and when the master medium is to be reused, the magnetic layer is reformed by a sputtering process or the like.
It is advantageous that the cleaning be performed in connection with the magnetic transfer or optical disk manufacture for each magnetic transfer (disk formation process), or after the magnetic transfer (formation) has been performed a predetermined number of times. Alternatively, it is preferable that the magnetic transfer master medium or the optical disk master medium be removed from the magnetic transfer apparatus or the disk formation apparatus, respectively, when transfer deficiencies or formation deficiencies due to adhered foreign matter occur, and then the cleaning process be performed. At this time, the efficiency obtained by preparing a plurality of master mediums and performing successive magnetic transfer or disk formation and cleaning processes in repetition is desirable.
According to the present invention described above, when a master medium having a magnetic layer formed over the uneven pattern surface of the substrate thereof is to be formed, because the pattern surface of the substrate has been cleaned, by use of a plasma discharge in an evacuated reactive gas environment, before the formation of the magnetic layer thereon, the material remaining from the protective layer can be completely removed when the magnetic layer is to be formed. Thereby the foreign matter originating in the protective layer and which becomes adhered to the surface of the master medium can be reduced, the occurrence of signal omissions due to contact deficiencies between the master medium and the slave medium can be reduced, magnetic transfers having a stable transfer quality can be performed and the reliability thereof improved, and the life span of the master medium can be extended.
Further, by providing a film on the data bearing surface of the magnetic transfer or optical disk master medium so as to remove the foreign matter adhered to the pattern surface of the master medium together with the film when the cleaning process is performed, the foreign matter including even minute particulate matter can be positively removed. Further, in particular for foreign matter that has become imbedded in the depression portions of the uneven pattern surface, because the width of the channels of the depression portions becomes wider upon the removal of the film from the side walls thereof, the foreign matter imbedded therein can be easily removed, the cause of transfer deficiencies and formation deficiencies are eliminated, the master medium becomes reusable, magnetic transfers having a stable transfer quality can be performed and the reliability thereof improved, and the life span of the master medium can be extended.